Three major classes of dental materials have been developed and used in dental practice: (1) inorganic/ceramic materials (e.g. amalgams and silicate cements), (2) organic polymers (e.g. polyacrylate resins) and (3) polymer composite materials in which fillers are mixed with a polymer at a macroscopic level. All of these materials, though succeeded in certain extent, have many inherent deficiencies in their dental applications. A novel class of dental materials is proposed, which covalently and uniformly incorporates organic polymers into an inorganic network at the molecular levels via a newly emerged sol-gel method. These molecular composite materials would have tailored combinations of the hardness of inorganic glasses and the toughness of organic polymers. Since the organic and inorganic components are covalently bonded and uniformly distributed in these new materials, there would be no interface problems as exist in the conventional dental composites. In this proposal, polyacrylates-SiO2-TiO2 systems are selected as typical examples for the initial studies with the view of taking advantage of well- established clinical applications of controlled molecular weights and desirable end groups such as -Si(OR)3, which is reactive in sol-gel reactions, will be synthesized by group-transfer polymerization method invented at Du Pont in 1983. Four types of telechelic polymers will be synthesized with one, two, three/four, or more reactive-Si(OR)3 groups. The telechelic polymers will then be introduced into the mixture of silicon and/or titanium tetraalkoxides followed b hydrolysis and crosslinking (densification) leading to the new molecular composite materials. This new class of materials will be characterized and tested. Depending on the nature and the ratio of the inorganic to the polymeric components, it should be superior in physical and chemical properties such as hardness, toughness, adhesion to tooth structure, resistance to wear and abrasion, stability, etc. over the existing dental materials.